The present invention relates to a chucking head, especially for hydraulic testing machines in which the test sample must be held without play and wherein static or dynamic material testing is performed on the test samples. Such chucking heads have at least two clamping pistons arranged opposite each other for holding a workpiece or test sample in a direction extending substantially at right angles to the direction defined by the longitudinal axis of the clamping pistons.
In material testing machines it is necessary that the clamping devices are suitable for all types of loads to be applied to the test sample, such as a tension force, pressure forces, and dynamic alternating loads or combinations thereof, including slowly varying dynamic loads. Where dynamic loads are used for testing, it is important that the chucking device does not permit any play in the direction in which the dynamic loads are applied. If such play exists, it is difficult to exactly define the load applied to the test sample or workpiece to be tested. Thus, undesirable loads might be applied which must be avoided. In addition, any play may result in a high wear and tear of the chucking device.
U.S. Pat. No. 3,908,449 granted on Sept. 30, 1975 discloses a chucking head operable without play and including two clamping pistons arranged opposite each other. In this known chucking head the play between the clamping pistons in the direction of load application is eliminated by using at least one additional piston which is effective on the clamping pistons in the direction of load application to the probe. The additional piston presses the clamping pistons against the cylinder walls of the clamping cylinders in such a manner that the play between piston and cylinder wall is eliminated in the direction of load application. This known apparatus is very effective, however, it leaves room for improvement, because the use of the additional piston for eliminating the play of the clamping pistons with its supply lines, actuating means and the like makes the clamping device more expensive.